Allow arbitrary strings as account tags in the FE

This mirrors the BE change:
https://gerrit-review.googlesource.com/c/gerrit/+/316442

Change-Id: I4d84039cc05095396a7e0082ee351fc3a06ab626

polygerrit-ui/app/constants/constants.ts

diff --git a/polygerrit-ui/app/constants/constants.ts b/polygerrit-ui/app/constants/constants.ts
index 84f1c15..2acad49 100644
--- a/polygerrit-ui/app/constants/constants.ts
+++ b/polygerrit-ui/app/constants/constants.ts
@@ -22,7 +22,6 @@
 import {DiffPreferencesInfo} from '../types/diff';
 import {EditPreferencesInfo, PreferencesInfo} from '../types/common';
 import {
-  AccountTag,
   AuthType,
   ChangeStatus,
   ConfigParameterInfoType,
@@ -42,7 +41,6 @@
 } from '../api/rest-api';
 
 export {
-  AccountTag,
   AuthType,
   ChangeStatus,
   ConfigParameterInfoType,
@@ -61,6 +59,10 @@
   SubmitType,
 };
 
+export enum AccountTag {
+  SERVICE_USER = 'SERVICE_USER',
+}
+
 export enum PrimaryTab {
   FILES = 'files',
   /**